1. Field of the Invention
The present invention relates to a polyester-type film that can be used as a base material for a label, poster, recording paper, wrapping paper, etc. In particular, it relates to a drawn polyester-type film containing a large number of fine voids, and a laminate using the film. The polyester-type film has enough surface strength to prevent its surface area from being peeled off when an adhesive tape sticks to its surface and is peeled off.
2. Description of the Prior Art
Synthetic papers which are made from synthetic resins as the main raw material are excellent in water resistance, dimensional stability due to moisture absorption, surface smoothness, gloss and distinctness when printing, and mechanical strength, compared to natural papers. Therefore, practical uses of synthetic papers have been developed in recent years by making use of these advantages.
As main raw material for the synthetic paper, polyethylene, polypropylene, polyesters, and the like are used. Especially, polyesters such as poly(ethylenetrephthalate) are excellent as raw material for synthetic paper because of high thermal resistance, and toughness, so it is possible to widely develop its uses.
The following processes have been proposed in a prior art for preparing a polyester film having similar functions to paper by using polyester as the main raw material:
(i) Containing a large number of fine voids inside the polyester film; PA1 (ii) Treating the smooth surface of the polyester film by a sandblast technique to make the surface rough; PA1 (iii) Treating the smooth surface of the polyester film by a chemical etching technique to make the surface rough; and PA1 (iv) Treating the smooth surface of the polyester film by laminating a matt agent together with a binder to make the surface rough. PA1 (1) providing a void-containing polyester-type film having excellent softness, lightness, and hiding power, compared to a drawn polyester-type film which does not contain any voids, substantially; PA1 (2) providing a void-containing polyester-type film whose surface is resistant to peeling; PA1 (3) providing a void-containing polyester-type film having suitable whiteness which makes it possible to print and type upon, distinctly; PA1 (4) providing a void-containing polyester-type film which makes it possible to write upon with a pencil or ball-point pen because of a large number of projections formed on the surface of the film; PA1 (5) providing a void-containing polyester-type film having a high surface strength by using single-layer extrusion equipment due to the fact that conventional laminating methods are not required to improve the surface strength; PA1 (6) providing a void-containing polyester-type film that can be used as a base material for a label, poster, recording paper, wrapping paper, etc., and from which these products, having durability, can be prepared; and PA1 (7) providing a laminate having an improved wetting property, and adhesiveness to ink and coating agents.
Among these processes, the aforementioned technique (i) has some advantages in that the film itself becomes light, and has suitable softness, thereby making it possible to print or transfer, distinctly.
In order to form fine voids inside the film, there has been proposed in the prior art a process comprising melt-kneading a polyester and a polymer which is not compatible with the polyester in an extruder, molding the kneaded material to form a sheet in which the polymer is dispersed in the polyester in the form of fine particles with this kneaded material, and then drawing the sheet to separate the fine particles from the polyester to form fine voids around the fine particles.
The polymers that can be used for the formation of the fine voids include polyolefin resins (for example, Japanese Laid-open Patent Publication No. 49-134755), polystyrene resins (for example, Japanese Patent Publications Nos. 49-2016 and 54-29550), and polyacrylate resins (for example, Japanese Patent Publication No. 58-28097), and so on. Among these polymers, polystyrene and polypropylene are preferred because fine voids are easily formed by using them.
However, a polyester film containing fine voids that is prepared by the aforementioned procedure (hereinafter referred to as a void-containing polyester film) has some disadvantages in that when an adhesive tape sticks to the surface of the film and is peeled off, the surface area is also peeled off. Also, when the surface of the void-containing film is worked to form an adhesive layer or laminated layer thereon for the preparation of a label or wrapping materials, the adhesive layer or laminated layer is susceptible to peeling from the film.
To solve these problems, a peeling-resistant layer is laminated on the surface of the film by a co-extrusion method, coat method, or laminate method. However, new equipment as well as complicated working processes are required to carry out these methods.